In a load-sensing hydraulic circuit including a plurality of actuators, the discharge flow rate of a pump is controlled such that the pressure difference between the pump pressure and the maximum load pressure among the load pressures of the respective actuators is constant. Generally speaking, in such a hydraulic circuit, a pressure compensation unit including a pressure compensation valve is provided for each actuator.
For example, Patent Literature 1 discloses pressure compensation units 100 as shown in FIG. 4. Each pressure compensation unit 100 includes a control valve 120, which controls the supply and discharge of a hydraulic fluid to and from an actuator 110. Each pressure compensation unit 100 also includes a shared pump line 101, an auxiliary pump line 102, a maximum load pressure line 103, and a shared tank line 104, which form passages extending across all the pressure compensation units.
The control valve 120 is connected to the shared pump line 101 by a supply line 111, connected to the actuator 110 by a pair of supply/discharge lines 114, and connected to the shared tank line 104 by a discharge line 115. The control valve 120 is also connected to a pressure compensation valve 130 by an upstream-side relay line 112 and a downstream-side relay line 113. The pressure compensation valve 130 is connected to the upstream-side relay line 112 by a first pilot line 131, and connected to a switching valve 140 by a second pilot line 132. The switching valve 140 is connected to the maximum load pressure line 103 by a first signal pressure line 161, and connected to the shared pump line 101 by a second signal pressure line 162.
The maximum load pressure line 103 is connected to the discharge line 115 by a relief line 151. The relief line 151 is provided with a relief valve 152, and also provided with a restrictor 153 positioned upstream of the relief valve 152. The switching valve 140 moves in accordance with the pressure difference between the maximum load pressure and the pressure of the relief line 151 at a position between the restrictor 153 and the relief valve 152.
If the maximum load pressure is lower than the setting pressure of the relief valve 152, the switching valve 140 is positioned in its neutral position, which is the upper position in FIG. 4, and leads the maximum load pressure to the pressure compensation valve 130. Accordingly, the pressure compensation valve 130 moves in accordance with the pressure difference between the pressure of the upstream-side relay line 112 and the maximum load pressure, and serves to keep constant the pressure difference between the upstream and downstream sides of the restrictor of the control valve 120 (i.e., the pressure difference between the pump pressure and the pressure of the upstream-side relay line 112). Therefore, even when the maximum load pressure varies, the flow rate of the hydraulic fluid supplied to the actuator 110 is kept constant.
On the other hand, if the maximum load pressure is higher than the setting pressure of the relief valve 152, the switching valve 140 shifts to a pressure-restricting position, which is the lower position in FIG. 4, and leads the pump pressure to the pressure compensation valve 130. Accordingly, the pressure compensation valve 130 blocks the upstream-side relay line 112 and the downstream-side relay line 113. Therefore, the load pressure of the actuator 110 can be kept to a desired pressure or lower. It should be noted that in a ease where a relief valve is provided on each of the supply/discharge lines 114 connected to the actuator 110, and the hydraulic fluid to the actuator is directly controlled by these relief valves, the flow rate of the hydraulic fluid flowing through the relief valves becomes significantly high, which causes a problem where a necessary flow rate for another actuator cannot be delivered.